Does the detection of X-ray emission from SN1998bw support its association with GRB980425?

We show that the recent identification of X-ray emission from SN1998bw is naturally explained as synchrotron emission from a shock driven into the wind surrounding the progenitor by a mildly relativistic shell ejected by the supernova, the existence of which was inferred earlier from radio observations. X-ray observations imply a shell energy E~10^{49.7}erg, and constrain the initial shell velocity \beta*c and normalized wind mass loss rate, \dot{m}=(\dot{M}/10^{-5}M_sun/yr)/(v_w/10^3 km/s), to satisfy \beta^3*\dot{m}~10^{-1.5}. The inferred energy is consistent with energy estimates based on radio observations provided \dot{m}~0.04, in which case radio observations imply \beta~0.8, consistent with the X-ray constraint \beta^3*\dot{m}~10^{-1.5}. While X-ray observations allow to determine the parameters characterizing the pre-explosion wind and the mildly relativistic shell ejected by SN1998bw, they do not provide evidence for existence of an off-axis "standard" GRB jet associated with SN1998bw, that may have produced GRB980425. However, as recently pointed out in (astro-ph/0310320), the lack of observational signatures typically expected to be produced by such an off-axis jet on a 1yr time scale, may be due to a low \dot{m}<0.1, which implies that an off-axis jet will become observable only on >10yr time scale.Comment: Minor changes. Accepted to ApJ

Library Perspectives, Issue 29, Fall 2003

This issue includes items about novelist Tracy Chevalier \u2784, the Robert M. Campbell (\u2734) Collection, The Holocaust Series exhibition, William Lloyd Garrison and The African Repository, and much more.https://digitalcommons.oberlin.edu/perspectives/1031/thumbnail.jp

Hypercritical Advection Dominated Accretion Flow

In this note we study the accretion disc that arises in hypercritical accretion of $\dot M\sim 10^8 M_{\rm Edd}$ onto a neutron star while it is in common envelope evolution with a massive companion. In order to raise the temperature high enough that the disc might cool by neutrino emission, Chevalier found a small value of the $\alpha$-parameter, where the kinematic coefficient of shear viscosity is $\nu=\alpha c_s H$, with $c_s$ the velocity of sound and $H$ the disc height; namely, $\alpha\sim 10^{-6}$ was necessary for gas pressure to dominate. He also considered results with higher values of $\alpha$, pointing out that radiation pressure would then predominate. With these larger $\alpha$'s, the temperatures of the accreting material are much lower, \lsim 0.35 MeV. The result is that neutrino cooling during the flow is negligible, satisfying very well the advection dominating conditions. The low temperature of the accreting material means that it cannot get rid of its energy rapidly by neutrino emission, so it piles up, pushing its way through the accretion disc. An accretion shock is formed, far beyond the neutron star, at a radius \gsim 10^8 cm, much as in the earlier spherically symmetric calculation, but in rotation. Two-dimensional numerical simulation shows that an accretion disc is reformed inside of the accretion shock, allowing matter to accrete onto the neutron star with pressure high enough so that neutrinos can carry off the energy.Comment: 6 pages, ApJ, submitte

X-ray Emission from the Type Ic Supernova 1994I Observed with Chandra

We present two high-resolution Chandra X-ray observations of supernova (SN) 1994I which show, for the first time, that the interaction of the blast wave from a Type Ic SN with its surrounding circumstellar material (CSM) can give rise to soft X-ray emission. Given a 0.3-2 keV band X-ray luminosity of L_x ~ 1 x 10^{37} ergs/s between six and seven years after the outburst of SN 1994I, and assuming the X-ray emission arises from the shock-heated CSM, we derive a pre-SN mass-loss rate of \dot{M} ~ 1 x 10^{-5} M_sun/yr (v_w/10 km/s). Combining the results with earlier ROSAT observations, we construct the X-ray lightcurve of SN 1994I. A best-fit X-ray rate of decline of L_x \propto t^{-s} with index s~1 and a CSM density profile of rho_csm \propto r^{-1.9\pm0.1} are inferred, consistent with what is expected for a constant mass-loss rate and constant wind velocity profile for the SN progenitor (rho_csm \propto r^{-2}).Comment: 5 pages, 3 figures, accepted for publication in the Astrophysical Journal Letter

Something Rational in the State of Denmark? The Case of an Outsider in the Cobden-Chevalier Network 1860-1875

We examine the case of an important outsider to the Cobden-Chevalier network of bilateral treaties in the second half of the nineteenth century. We attempt to explain this through a study of the structure of Danish trade and protection. We demonstrate, in contrast to previous accounts that have considered Danish trade policy somewhat irrational, that Denmark was right to remain outside. She had little to gain from concluding treaties, since her main trading partners offered free trade for her exports, agricultural goods, and she needed her own tariffs for revenue purposes.bilateral treaties; Cobden-Chevalier network; Denmark

Magnetic anomalies in Nd6Co(1.67)Si3: Surprising first order transitions in the low-temperature isothermal magnetization

We present the results of magnetic measurements on Nd6Co(1.67)Si3, a compound recently reported to crystallize in a hexagonal structure (space group P6_3/m) and to undergo long range magnetic ordering below 84 K. The results reveal that the magnetism of this compound is quite complex with additional magnetic anomalies near 50 and 20 K. There are qualitative changes in the isothermal magnetization behavior with the variation of temperature. Notably, there is a field-induced spin reorientation as the temperature is lowered below 20 K. A finding we stress is that this transition is discontinuous for 1.8K in the virgin curve, but the first order character appears only after a field-cycling for a narrow higher temperature range near 5 K. Thus, this compound serves as an example for the stabilisation of first-order transition induced by magnetic-field-cycling. The issues of 'Phase co-existence' and 'meta-stability' after a field-cycling at low temperatures in this compound are also addressed

The Vela Pulsar Wind Nebula at 6cm

Observations using the Australia Telescope Compact Array at a wavelength of 6 cm have uncovered the radio counterpart to the compact X-ray nebula surrounding the Vela pulsar. Two lobes were found oriented about the spin axis of the pulsar, starting at the edge of X-ray emission, they extend to three times the size. The northern lobe has a bright, defined edge and an integrated flux of 0.14 Jy, while the southern lobe of 0.12 Jy is more diffuse.Comment: 4 pages, 3 figures. To appear in "Neutron Stars in Supernova Remnants" (ASP Conference Proceedings), eds P. O. Slane and B. M. Gaensle

Observations of supernova 1979c in M 100

The IUE observations of supernova 1979c in M 100 are presented and discussed. The main results are: (1) the bulk of the energy is in the form of continuous emission which is radiated by the main SN envelope; (2) the absorption features originate mostly in both the disks and the haloes of our Galaxy and M 100; and (3) the emission lines are produced in a highly ionized shell which has a radius greater than twice the radius of the main envelope and consists of compressed circumstellar material in which the abundance ratio N/C is about 30 times higher than solar

The "sigma" problem of the Crab pulsar wind

The conversion of the Crab pulsar wind from one dominated by Poynting flux close to the star to one dominated by particle-born energy at the termination shock is considered. The idea put forward by Coroniti (1990) and criticised by Lyubarsky & Kirk (2001) that reconnection in a striped wind is responsible, is generalised to include faster prescriptions for the a priori unknown dissipation rate. Strong acceleration of the wind is confirmed, and the higher dissipation rates imply complete conversion of Poynting flux into particle-born flux within the unshocked wind.Comment: 4 pages, to appear in "Young Neutron Stars and Their Environments" (IAU Symposium 218, ASP Conference Proceedings), eds F. Camilo and B. M. Gaensle